The spinal cord is a central structural element of the human skeleton comprising a plurality of vertebrae, which are arranged one above one another for the transfer of loads and are connected to one another with articulation to allow movements. The vertebrae of the spinal cord are not identical but instead differ in shape depending on their arrangement on the spinal cord. However, they have a few things in common. For example, each vertebra has a solid vertebral body with two osseous protrusions (pedicles) protruding laterally and to the rear, each in turn being connected in its rear portion via a bony arch. In the connecting area this bony arch is designed as a broad plate (lamina) and has a spinal protrusion extending to the rear at the center. The spinal protrusion (spinal process) as well as two additional transverse protrusions on the lateral surfaces of the pedicle form attachment points for muscles and ligaments. In the area where the pedicles develop into the broad lamina, one upper joint protrusion and one lower joint protrusion are arranged on each side of the vertebrae. They each form part of a facet joint having a neighboring lower or upper vertebra. In addition, it is provided that for transfer of load on the vertebrae, an intervertebral disc is arranged between the vertebral bodies of neighboring vertebrae, filling up the interspace between the relatively flat cover surfaces of neighboring vertebral bodies. The region bordered by the back sides of the vertebral body and the bony arch (vertebral arch) forms a cavity in which nerve fibers running parallel to the spinal cord are accommodated.
Back pain or back ache often occurs due to degeneration of the spinal cord. One of the main causes for back pain is the interaction between two neighboring vertebrae. This relates in particular to intervertebral discs as one of the main causes, but also in a substantial number of cases the pathology also involves at least the facet joints. Because of wear or disease, the articulated connection of two neighboring vertebrae created for the facet joints may be damaged. This may lead to restricted movement, pain or even loss of mobility. Various approaches have become known for treatment. In particular a definite improvement can be achieved by stabilizing the facet joint. In many fields, this is done by immobilizing the facet joint by a fixed connection. We speak here of fusion of the facet joint.
WO 2009/094629 A1 has disclosed a fusion implant comprising long bone screws which are screwed through both of the facets forming a facet joint. This screw is designed as a compression screw, tightening the cooperating halves of the facet joints so that the joint is immobilized. To be able to transfer the required forces to the vertebral body, the screw head is provided with a separate supporting sleeve arranged in a pivotably mobile manner. With its pivotable mobility, the transfacetal compression screw may assume various angular positions to the supporting plane determined by the supporting sleeve. We speak here of a polyaxial arrangement of the transfacetal screw. A separate screw is provided for each of the two facet joints of a vertebral body (on the left side or on the right side). The known fusion implant offers the advantage of relatively simple implantability because it has small dimensions and therefore can be implanted even in minimally invasive surgery. However, this known fusion implant requires a relatively strong and intact bone structure on the vertebral body, in particular in the area of the supporting surface of the pivotably movable collar around the screw head.
US 2005/0192572 A discloses a fusion implant which also provides polyaxially guided transfacetal facet screws for fusion of the two joint halves of a facet joint. In contrast with the implantation arrangement described previously, this embodiment additionally has a traverse piece on which the two transfacetal fusion screws (for the right and left facet joints of a vertebral body) are guided via sliding connectors. The sliding connectors are designed so that they are fixed in their position by clamping forces which occur on insertion and tightening of the facet screw. The advantage of this implantation arrangement is that a stable positioning of the one facet screw in relation to the facet screw on the other side of the vertebral body is achieved. However, there is no control of the absolute position of the fusion screws with respect to the vertebral body. Furthermore, this implantation arrangement also requires a relatively strong and intact bone structure of the vertebral body.